Internal combustion engine misfire conditions occur when an air-fuel mixture in an engine cylinder is improperly consumed. Misfire conditions may be caused by a missing or improperly timed delivery of fuel, insufficient delivery of cylinder intake air, or a missing or improperly timed cylinder spark event. Reduced engine performance and fuel economy or increased engine emissions may result from a misfire condition. Misfire conditions must be accurately diagnosed to avoid unnecessary application of misfire treatment procedures.
Misfire detection procedures have been proposed for measuring sub-cyclic engine speed as communicated by passage of teeth or notches about an engine output shaft and by detecting misfire conditions following detection of a pattern of short speed decreases. Such procedures typically only operate when certain entry conditions are present. Misfires occurring when the entry conditions are not present are not detected. Such procedures may require addition of costly hardware on the vehicle, adding to vehicle cost and adding sources of potential fault conditions. Such procedures further may be computationally intensive, adding significantly to engine or vehicle controller throughput burden. Such procedures may still further not compensate for the corrupting effect of time varying disturbances and noise on engine parameter signals, which can reduce diagnostic integrity. Those procedures that do attempt to compensate for disturbances only learn and correct for tooth machining errors. The learning requires specific engine operating conditions. If the conditions are not present, the learning and correction does not take place and the diagnostic cannot function accurately.
The signal to noise ratio of conventional misfire detection proposals using sub-cyclic engine speed information is typically low in certain engine operating ranges, such as in low engine load, medium to high engine speed ranges. This is because the change in engine speed following a misfire condition is commonly small in these ranges and the level of noise and disturbances corrupting the engine speed signal is typically large. Several factors tend to increase the magnitude of the noise and disturbances. These factors include runout, cylinder to cylinder variation, torsionals, driveline noise, output shaft tooth or notch machining error, rough driving surface disturbances, and measurement resolution limitations. Current misfire diagnostic proposals are therefore prone to misdiagnosis, and especially in certain, frequently occurring engine operating ranges. Misdiagnosis can lead to application of unnecessary, costly and time-consuming fault treatment procedures.
It would therefore be desirable to provide for accurate misfire detection over all engine operating conditions that minimizes diagnostic sensitivity to noise, transients and disturbances, without adding significant cost and without overburdening the system.